docs: def getExpiresIn(self, now=None):
          """
          This returns the number of seconds this association is still
          valid for, or C{0} if the association is no longer valid.
  
  
          @return: The number of seconds this association is still valid
              for, or C{0} if the association is no longer valid.
  
          @rtype: C{int}
          """
          if now is None:
              now = int(time.time())
  
          return max(0, self.issued + self.lifetime - now)